A known work dock is described in DE-A-3 720 239. It is used to carry out work on the horizontal tail unit and the rudder unit of an aircraft. It comprises a single framework, which is laterally displaceable on rails and has two vertically adjustable work platforms lying one above the other and extending over the entire length of the framework. For docking and undocking the aircraft, said frameworks are displaced in a lateral direction far enough to allow the rudder units to be moved through between them.
For minor maintenance and repair jobs to the outer skin of aircraft, adjustable mobile work cages are also used, by means of which a mechanic may be brought to the desired point of the outer skin. Such mobile work cages are very versatile but they are less suitable for work carried out on the entire aircraft, such as major overhauls, involving a great many--typically 80 to 100--mechanics. If mobile work cages are used for such work, only a total of at most 10 work cages may operate on the aircraft, each carrying 1 or 2 mechanics and additionally requiring a driver for reliable avoidance of collisions. A further drawback of such mobile work cages is that the work cage, which is carried by a long jib, is not adjustable without backlash or jolting, this being a drawback when it comes to carrying out surface work cleanly (e.g. painting).
For this reason, fixed scaffolds are often used for major overhauls of aircraft. The entire aircraft is scaffolded like a building and on the work platforms supported by the scaffolding the various workers may move independently of one another to desired points of the outer skin. However, it takes on the whole about 80 to 90 hours to erect and remove the scaffolding around an aircraft and during this period the maintenance crew are totally or largely idle.
It has also already been proposed to maintain the outer skin of aircraft from work cages suspended via vertically adjustable telescopes from crabs travelling along rails which are provided on the roof of the building. However, a typical work cage unit of this type weighs around 15 tonnes and, even if only the same total number of work cages achievable with mobile work cages were to be provided, the roof structure would be loaded with an additional weight of 150 tonnes. Since the crabs still have to be able to move beyond the highest point of the aircraft (vertical rudder) and the crab and the rails for the crab take up some room, a hangar accommodating such a dock additionally has to be built very high throughout its area. It is therefore not only the high loading capacity requirements of the roof structure which make such a hangar expensive, but also the building volume, with the large volume also increasing routine maintenance costs.